Power transformers and reactors are pieces of equipment widely used in medium, high and extra-high voltage electric power generation, transmission and distribution systems. These devices frequently use for insulation and heat removal some type of oil, which can be mineral (petroleum byproduct), vegetal (from soy bean, sunflower or other) or silicon, for example.
Transformers and reactors have copper coils that are wrapped in paper. The entire active part—comprised of a core and coils—is then immersed in insulating oil to impregnate the paper and thus ensure the electric isolation of the assembly, and to cool the coils with the oil circulating in heat radiators.
Thus, for safe operation of the equipment, it is essential for the active part to be permanently immersed in insulating oil. However, temperature variations during equipment operation due to variations in ambient temperature and heating caused by the electric current, make the oil dilate and contract, thus making its volume vary and increasing and reducing oil level.
In order to guarantee the active part is permanently immersed in insulating oil during all operating conditions, the transformer or reactor is equipped with an oil expansion tank, also called a conservation tank, installed above the equipment's main tank and connected to it by piping. The function of the conservation tank is to provide room for the oil level to vary in its interior, rising with increases in temperature and falling with decreases. The volume of the conservation tank is calculated in such a manner that the main tank, where the active part is located, will always be completely full of oil, even at the lowest temperatures, and the oil will never overflow, even at the highest expected temperatures.
Therefore, given the importance of oil level for safe operation of the equipment, it is necessary to continuously measure it in order to readily detect any level lower than minimum tolerances, thus preventing a short circuit due to lack of oil in the active part, and to warn of environmental contamination with oil due to eventual leaking. Likewise, measuring the oil level permits detecting higher than expected levels due to filling equipment with excess oil; for example, warning of overflow risks and contamination of the environment.
In modern transformers and reactors, the expansion tank is also equipped with a rubber membrane or bag that impedes the oil's direct contact with the air; however, without impeding any variation in oil level, since the membrane or bag is flexible, rising and falling in accordance with the oil level. The upper part of the rubber membrane or bag is in contact with the external environment through air piping and an air dehumidifying mechanism. This avoids pressure or vacuum in the equipment tank due to expansion or contraction of oil.
In the current state of the art, the oil level measuring system is comprised of an oil level indicator that operates using a float located in the lower part of the rubber membrane or bag, so that the float rises and falls accompanying the movement of the membrane or bag, which in turn accompanies the increase or reduction in oil level. The float is coupled to an oil level indicator by a rod that moves a mechanism, which in turn activates the pointer, indicating current oil level at a graduated scale.
This arrangement can be observed for example in U.S. Pat. Nos. 7,191,648 and 6,708,562.
When the oil level reaches critical conditions, such as low, very low, high or very high levels, movement of the mechanism closes one or more electrical contacts, which are used to activate a visual or sonorous alarm in the installation control room.
The state of the art for the oil level indication system presents some disadvantages observed in practice, which are:                Because it is a mechanically activated system, it is subject to mechanical failures such as jamming of the mechanism;        In order to prevent mechanical failures, it is necessary to have preventive maintenance for lubrication, for example;        With the up and down movement of the oil level, the rubber of the membrane or bag may present undulations or folds in which the float may get caught, leading to the imprecise indication of oil level;        In extreme cases, the above described fact may cause the float and/or its rod to perforate the membrane or bag, putting the oil in contact with oxygen and humidity in the air and causing the accelerated aging of the isolation paper on the coils by oxidation and hydrolysis;        Leak detection cannot be conducted immediately since the loss of oil to the environment may be masked by an increase in level due to temperature increases, so the alarm contacts for low levels will only be activated after a considerable volume of oil has been disposed of in nature;        Likewise, detection of a transformer that has been overfilled with oil cannot be done immediately since the existence of excess oil can be masked by the reduction in oil level due to low temperatures when the transformer is shut off. Thus, the contacts will only be activated some time after the transformer has been energized, when the temperature rises, causing the forced shut down of the equipment for removal of oil or the leaking of excess oil into the environment.        